NIF: The "Crown Joule" of Laser Science

The National Ignition Facility (NIF) is the world's largest laser. NIF's 192 intense laser beams can deliver to a target more than 60 times the energy of any previous laser system. NIF became operational in March 2009 and is capable of directing nearly two million joules of ultraviolet laser energy in billionth-of-a-second pulses to the target chamber center.

Aerial View of the National Ignition Facility

Three football fields could fit inside the NIF Laser and Target Area Building.

When all that energy slams into millimeter-sized targets, it can generate unprecedented temperatures and pressures in the target materials—temperatures of more than 100 million degrees and pressures more than 100 billion times Earth's atmosphere. These conditions are similar to those in the stars and the cores of giant planets or in nuclear weapons; thus one of the NIF & Photon Science Directorate's missions is to provide a better understanding of the complex physics of nuclear weapons (see National Security). Researchers can also explore basic science, such as astrophysical phenomena, materials science, and nuclear science (see Understanding the Universe). NIF's other major mission is to provide scientists with the physics understanding necessary to create fusion ignition and energy gain for future energy production (see Energy for the Future).

NIF encompasses three interconnected buildings: the Optics Assembly Building, the Laser and Target Area Building, and the Operations Support Building (see Virtual Tour). Inside the Optics Assembly Building large, precision-engineered laser components are assembled under stringent cleanroom conditions into special modules called line replaceable units, or LRUs, for installation into the laser system. Laser Bay 2, one of NIF's two laser bays, was commissioned on July 31, 2007. The Laser and Target Area Building houses the 192 laser beams in two identical bays. Large mirrors, specially coated for the laser wavelength and mounted on highly stable ten-story-tall structures, direct the laser beams through the "switchyards" and into the target bay. There they are focused to the exact center of the ten-meter-diameter, concrete shielded, one-million-pound target chamber. Construction of all the buildings and supporting utilities was completed in September 2001. All 192 enclosures for laser beams were completed in 2003.

Operation of NIF's extraordinarily energetic laser beams requires that everything in the beam's enclosures remain perfectly clean at all times. Any bit of debris, oil, or other wayward material could cause the intense light to damage the optics (see Optics). The space inside the beam enclosures typically exceeds the cleanliness of a semiconductor or pharmaceutical manufacturing plant.

Extraordinary Precision

Every NIF experimental shot requires the coordination of up to 60,000 control points for electronic, high voltage, optical, and mechanical devices—motorized mirrors and lenses, energy and power sensors, video cameras, laser amplifiers, and diagnostic instruments. Achieving this level of precision requires a large-scale computer control system as sophisticated as any in government service or private industry (see Integrated Computer Control System). The meticulous orchestration of these parts will result in the propagation of 192 separate nanosecond-long (billionth of a second) bursts of light over a one-kilometer path length. The 192 separate beams must have optical pathlengths equal to within nine millimeters so that the pulses can arrive within 30 picoseconds (trillionths of a second) of each other at the center of the target chamber. Then they must strike within 50 micrometers of their assigned spot on a target the size of a pencil eraser. NIF's pointing accuracy can be compared to standing on the pitcher's mound at AT&T Park in San Francisco and throwing a strike at Dodger Stadium in Los Angeles, some 350 miles away. Because the precise alignment of NIF's laser beams is extremely important for successful operation, the requirements for vibrational, thermal, and seismic stability are unusually demanding. Critical beampath component enclosures (generally for mirrors and lenses), many weighing tens of tons, were located to a precision of 100 microns using a rigorous engineering process for design validation and as-installed verification.

Why Are There 192 Beams?

Imagine trying to squash a water balloon with two hands. No matter how hard you try to spread your fingers evenly over the surface of the balloon, it will still squirt out between your fingers. Many more fingers would be needed to compress the balloon symmetrically. Earlier high-energy lasers were used to study the conditions required to compress tiny spherical capsules to fractions of their initial diameter while still maintaining the capsule's symmetry—a crucial requirement if NIF is to achieve fusion ignition. NIF's designers arrived at 192 focused spots as the optimal number to achieve the conditions that will ignite a target's hydrogen fuel and start fusion burn.

A Variety of Experiments

Not all experiments on NIF need to produce fusion ignition. Researchers are planning many other types of experiments that will take advantage of NIF's tremendous energy and flexible geometry in non-ignition shots. Non-ignition experiments will use a variety of targets to derive a better understanding of material properties under extreme conditions. These targets can be as simple as flat foils or considerably more complex. By varying the shock strength of the laser pulse, scientists can obtain equation-of-state data that reveal how different materials perform under extreme conditions for stockpile stewardship and basic science. They also can examine hydrodynamics, which is the behavior of fluids of unequal density as they mix.

NIF experiments also will use some of the beams to illuminate "backlighter" targets to generate an x-ray flash. This allows detailed x-ray photographs, or radiographs, of the interiors of targets as the experiments progress. In addition, moving pictures of targets taken at one billion frames a second are possible using sophisticated cameras mounted on the target chamber. These diagnostics can freeze the motion of extremely hot, highly dynamic materials to see inside and understand the physical processes taking place (see Diagnostics). As construction of the 48 "quads" of four beams each proceeded, many shots were already being performed using the first quad of beams (see NIF Early Light). Following NIF's completion and dedication in 2009, experiments using all 192 laser beams demonstrated NIF's ability to create the conditions needed for ignition experiments beginning in 2010.

Technicians adjust the target positioner inside the NIF Target Chamber.

New Technologies Make NIF Possible

Amplifying NIF's beams to record-shattering energies, keeping the highly energetic beams focused, maintaining cleanliness all along the beam's path, and successfully operating this enormously complex facility—all required NIF's designers to make major advances in existing laser technology as well as to develop entirely new technologies (see The Seven Wonders of NIF). Innovations in the design, manufacture, and assembly of NIF's optics were especially critical (see Optics).

More Information

"The National Ignition Facility Comes to Life," Science & Technology Review, September 2003

"The Journey Into a New Era of Scientific Discoveries,"Science & Technology Review, April/May 2009